In recent years, as a transmission system in which a plurality of transmission apparatuses are connected to each other, for example, there is not only a linear type and a ring type but also a mesh type. The transmission system includes a master station provided with a transmission apparatus which distributes a clock for synchronization (synchronization clock) from a clock source, and a slave station provided with a transmission apparatus which is subordinately synchronized with the synchronization clock from the master station and performs each operation based on the subordinately synchronized clock. In addition, in the transmission system of the master slave synchronization type, in order not to cause an unneeded clock variation, generally, a line switched due to a line failure is not switched back when recovered from the failure.
However, in the transmission system of the master slave synchronization type, in a case where a line failure occurs between the master station and the slave station, each slave station may not receive the synchronization clock from the master station, and may receive, as a synchronization clock, a clock which is synchronized with a clock output from the slave station when the synchronization clock is recovered. Therefore, a synchronization clock loop may occur. In the transmission system, in a case where the synchronization clock loop occurs, a frequency difference of a synchronization clock of each transmission apparatus increases, and thus communication quality between the transmission apparatuses deteriorates.
As a method of minimizing the occurrence of the synchronization clock loop, there is a method of using an extended SSM as an extension of a synchronization status message (SSM) which is defined in International Telecommunication Union (ITU-T) G.707 and G.704. The SSM is a message for transmitting quality of a synchronous network timing reference.
For example, in a case where the extended SSM is used in a ring type transmission system in which clocks are supplied between transmission apparatuses, a transmission apparatus inserts an ID for identifying the transmission apparatus into the extended SSM in a transmission signal, and transmits the signal into which the ID is inserted, to other transmission apparatuses. In addition, each transmission apparatus determines whether or not an ID extracted from the extended SSM in a transmitted signal received from other transmission apparatuses is the same as an ID thereof. The transmission apparatus determines that a synchronization clock loop occurs in a case where the ID in the extended SSM is the same as the ID thereof. In the ring type transmission system, it is possible to check whether or not a clock loop occurs.
Japanese Laid-open Patent Publication No. 2004-032175 is an example of the related art.